Precision-casting core, precision-casting core manufacturing method, and precision-casting mold

ABSTRACT

A precision-casting core body is formed by mixing and sintering silica particles and silica fume. The resistance of the mixture of silica fume decreases during heating injection molding, and hence the fluidity is improved by the addition of silica fume. As a result, since the fluidity is improved, it is possible to decrease an injection molding pressure when a core is manufactured. Further, the mixture can be applied to a thin compact and a complex compact with improved fluidity.

FIELD

The present invention relates to a precision-casting core, a precision-casting core manufacturing method, and a precision-casting mold.

BACKGROUND

As a precision-cast product, for example, a turbine blade used in a gas turbine is known. In the gas turbine, a working fluid is heated by a burner so as to be a high-temperature/high-pressure working fluid, and the turbine is rotated by the working fluid. That is, the working fluid compressed by a compressor is heated by the burner so as to increase the energy of the working fluid, the energy is recovered by the turbine so as to generate a rotation force, and hence electric power is generated by the rotation force. The turbine is provided with a turbine rotor, and the outer periphery of the turbine rotor is provided with at least one gas turbine blade.

Here, since a turbine blade of a jet engine or a land gas turbine is exposed to a high temperature, a complex cooling structure (an air hole) through which a cooling medium (air) flows is provided therein. In order to form such an internal cooling structure, a core (formed of silica) having the same shape as a cooling medium flow passage is disposed (provided) inside a mold so as to perform a casting process and a cooling process. Accordingly, a cast metal product can be obtained. At this time, an outer shape can be obtained when the mold is broken. However, since the core is left therein, the core is generally removed by alkali (NaOH or KOH) through dissolving and removing.

Accordingly, since the core needs to be dissolved by alkali, a silica material (SiO₂) is used (Patent Literature 1).

Here, a precision-casting core can be obtained by molding a silica material such as melted silica (SiO₂) through injection molding or slip casting and performing a heat treatment thereon.

The injection molding method is a method of obtaining a compact by kneading ceramic powder and wax, injecting a material obtained by heating and melting the wax into a metal mold, and cooling and hardening the material.

Further, the slip casting method is a method of preparing slurry by mixing ceramic powder with water or the like, pouring the slurry into a mold formed of a material such as gypsum absorbing a solution, and drying the slurry so as to obtain a desired molded shape.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Laid-open Patent Publication 6-340467

SUMMARY Technical Problem

Incidentally, since the silica material is generally obtained by milling an ingot formed by fusing quartz, a problem arises in that the particle size of the material is comparatively coarse. Such a material is molded in a core shape by a method such as injection molding and is subjected to a heat treatment (baking). However, since the particle size is coarse, the strength of the sintered body is low.

Further, since the fluidity of the material is low during injection molding, a problem arises in that a high injection pressure is needed.

In addition, since the existing core is mainly manufactured in consideration of alkali solubility, a problem arises in that the high-temperature strength of the core is low.

Further, in the injection molding method, a plurality of holes is formed in the surface of the core which is sintered after molding. As a result, a problem arises in that the strength is low and the core is broken from the holes as the start points during casting.

Accordingly, there has been a desire for the precision-casting core the high-temperature strength of which is improved.

The invention is made in view of the above-described circumstance, and an object thereof is to provide a precision-casting core the fluidity and the high-temperature strength of which are improved, a precision-casting core manufacturing method, and a precision-casting mold.

Solution to Problem

According to a first aspect of the present invention to solve the above mentioned problems, there is provided a precision-casting core in which a precision-casting core body is formed by mixing and sintering silica particles and silica fume.

According to a second aspect, in the first aspect, there is provided the precision-casting core, wherein a coating layer is formed on the surface of the sintered precision-casting core body.

According to a third aspect, there is provided a precision-casting mold used to manufacture a cast metal, comprising the precision-casting core of the first or second aspects having a shape corresponding to a cavity inside the cast metal and an outer mold corresponding to the shape of the outer peripheral surface of the cast metal.

According to a fourth aspect, there is provided a precision-casting core manufacturing method comprising immersing a sintered body of a precision-casting core body mainly including silica particles into a coating material including a silica material and an alumina material, drying the sintered body and heating the sintered body so as to form a coating layer on the surface of the precision-casting core body.

According to a fifth aspect, in the fourth aspect, there is provided the precision-casting core manufacturing method, wherein the silica material is silica sol and the alumina material is alumina sol.

Advantageous Effects of Invention

According to the invention, it is possible to improve the fluidity of the compound prepared by adding silica fume of spherical ultrafine particles to silica particles having coarse particle sizes. Thus, it is possible to decrease an injection molding pressure when the core is manufactured.

Further, since the coating layer of two kinds of silica materials having different particle diameters is formed on the surface of the sintered precision-casting core body, the holes formed in the surface during sintering are sealed. Accordingly, there is an effect that the breakage of the core is prevented during casting in that the strength of the core is improved and the holes are sealed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a relation between strength and adding amount of silica fume in Example 2.

FIG. 2 is a diagram illustrating a relation between strength and adding amount of silica fume in Example 3.

FIG. 3 is a flowchart illustrating an example of a process of a casting method.

FIG. 4 is a flowchart illustrating an example of a process of a mold manufacturing method.

FIG. 5 is a schematic diagram illustrating a core manufacturing process.

FIG. 6 is a schematic perspective view illustrating a part of a metal mold.

FIG. 7 is a schematic diagram illustrating a wax pattern manufacturing process.

FIG. 8 is a schematic diagram illustrating a configuration of applying slurry to a wax pattern.

FIG. 9 is a schematic diagram illustrating an outer mold manufacturing process.

FIG. 10 is a schematic diagram illustrating a part of a process of the mold manufacturing method.

FIG. 11 is a schematic diagram illustrating a part of a process of the casting method.

FIG. 12 is a schematic diagram illustrating a core manufacturing process according to Example 2.

FIG. 13 is a cross-sectional configuration diagram of a precision-casting core.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the invention will be described in detail with reference to the drawings. Furthermore, one invention is not limited to the description below. Further, the components to be described below include a component which can be easily supposed by the person skilled in the art, a component which has substantially the same configuration, and a so-called equivalent component.

EXAMPLE 1

The precision-casting core according to the invention is obtained by mixing and sintering silica particles and silica fume (having a particle diameter of 0.15 μm) so as to form the precision-casting core body.

Here, silica particles are formed by, for example, melted silica (SiO₂) such as silica sand and silica flour.

The core body is manufactured by a known method in which silica fume (having a particle diameter of 0.15 μm) is added to silica particles, for example, silica sand (220 mesh (20 to 70 μm) or 350 mesh (20 to 40 μm)) or a mixture prepared by mixing silica flour (for example, 800 mesh (10 to 20 μm)) and silica sand (for example, 220 mesh or 350 mesh) at the weight ratio of 1:1, wax is added thereto, and the resultant is heated and kneaded so as to obtain a compound.

Here, it is desirable that silica fume have a particle diameter of 0.05 to 0.5 μm.

Here, as illustrated in the examples to be described below, it is desirable that silica fume mixed with silica particles be 5 wt % or more, desirably 10 wt % or more, and more desirably 20 wt % or more. This is because the strength of the core is not so much improved when the content is smaller than 5 wt %.

A core compact is obtained by injection molding the obtained compound.

Subsequently, a core body is obtained by performing a decreasing treatment to, for example, 600° C. and a sintering treatment at, for example, 1,200° C.

It is possible to improve the fluidity of the compound prepared by adding silica fume of spherical ultrafine particles to silica particles having coarse particle sizes. Thus, it is possible to decrease an injection molding pressure when the core is manufactured.

TEST EXAMPLE 1

A test example that verifies the effect of the fluidity of the invention will be described.

A comparative test was performed on the fluidity of the compound in order to check improvement in injection moldability.

(Thermoplastic) wax of 10 wt % was added and kneaded into silica particles (for example 220 mesh) so as to obtain a compound as a comparative compound.

Silica fume was mixed with silica particles (for example 220 mesh) at the weight ratio of 9:1, and (thermoplastic) wax of 10 wt % was added and kneaded thereinto so as to obtain a compound as a sample compound.

The comparative compound not including silica fume in the related art and the sample compound including silica fume were respectively injected into an injection molding machine so as to be heated and injected. At that time, the pressure values thereof were compared with each other.

When the pressure value of the comparative compound only including silica particles in the related art is regarded as 100, the pressure value of the sample compound including silica fume of 10 wt % corresponds to 85, and it was verified that the resistance in the sample compound decreased and the fluidity was improved by the addition of silica fume.

As a result, since the fluidity of the sample compound is improved, it is possible to decrease an injection molding pressure when the core is manufactured. Further, the compound can be applied to a thin compact and a complex compact with improved fluidity.

TEST EXAMPLE 2

A test example that shows the effect of the strength of the invention will be described.

In the test example, sample compounds were obtained by adding wax to mixtures in each of which silica fume of 10 wt %, 20 wt %, 30 wt %, and 40 wt % was added respectively to silica sand (220 mesh: 20 to 70 μm) and heating and kneading the mixtures. Here, “RD-120” (product name) manufactured by Tatsumori Ltd. was used as silica sand.

A compact was obtained by injection molding each of the obtained sample compounds.

As each test sample, a sample having a width of 30 mm, a length of 200 mm, and a thickness of 3 mm was obtained.

Next, a test sample for a core body of each of the sample compound was obtained by performing a decreasing treatment to 600° C. and a sintering treatment at 1,200° C.

The strength of the obtained test samples were measured.

Here, the strength test was performed based on “Bending Strength of Ceramics (1981)” of JIS R 1601.

The test result of Test Example 2 is illustrated in FIG. 1. FIG. 1 is a diagram illustrating a relation between strength and adding amount of silica fume in Test Example 2.

As illustrated in FIG. 1, the bending strength increased with an increase in the adding amount of silica fume to 20 wt %. Furthermore, the strength improvement degree was small at the adding amount of silica fume of 20 wt % or more.

As a result, since there was a concern that the time necessary for dissolving the core in an alkali solution might increase in that the mixed silica fume was fine particle and was further compactly formed when the adding amount was too much, it was verified that the adding amount of silica fume needed to be 5 wt % to 30 wt % and desirably 10 wt % to 30 wt % in order to improve the strength of the core.

In this way, since the compound is prepared by adding silica fume of spherical ultrafine particles to silica particles having coarse particle sizes, the core structure becomes compact and hence the bending strength is improved.

TEST EXAMPLE 3

A bending test was performed similarly to Test Example 2 except that silica sand particle of 350 mesh (20 to 40 μm) was used.

The test result of Test Example 3 is illustrated in FIG. 2. FIG. 2 is a diagram illustrating a relation between the strength and the adding amount of silica fume in Test Example 3.

As illustrated in FIG. 2, since silica sand was finer than that of Test Example 2, the initial bending strength value was high. Further, the bending strength increased with an increase in the adding amount of silica fume to 20 wt %. Furthermore, the strength improvement degree was small at the adding amount of silica fume of 30 wt % or more. It was verified that the adding amount of silica fume needed to be 5 wt % to 30 wt % and desirably 10 wt % to 30 wt % in order to improve the strength of the core in FIG. 2.

Hereinafter, a casting method using a mold having the precision-casting core of the invention disposed therein will be described.

FIG. 3 is a flowchart illustrating an example of a process of the casting method. Hereinafter, the casting method will be described with reference to FIG. 3. Here, the process illustrated in FIG. 3 may be totally automatically performed or may be performed by the operator operating each device dedicated for the process. In the casting method of the embodiment, a mold is manufactured (step S1). The mold may be manufactured in advance or may be manufactured for each casting.

Hereinafter, the mold manufacturing method of the embodiment performed by the process of step S1 will be described with reference to FIGS. 4 to 10. FIG. 4 is a flowchart illustrating an example of the process of the mold manufacturing method. Here, the process illustrated in FIG. 4 may be totally automatically performed or may be performed by an operator operating each device dedicated for the process.

In the mold manufacturing method, a core is manufactured (step S12). The core has a shape corresponding to the cavity inside the cast metal manufactured by the mold. That is, since the core is disposed at a portion corresponding to the cavity inside the cast moral, it is possible to suppress metal as the cast metal from flowing thereinto during casting. Hereinafter, the core manufacturing process will be described with reference to FIG. 5.

FIG. 5 is a schematic diagram illustrating the core manufacturing process. In the mold manufacturing method, as illustrated in FIG. 5, a metal mold 12 is prepared (step S101). The metal mold 12 is formed so that an area corresponding to the core is formed as a cavity. A portion formed as the cavity of the core becomes a convex portion 12 a. Furthermore, although it is illustrated in the cross-section of the metal mold 12 in FIG. 5, the metal mold 12 is formed so that an area other than an opening for injecting a material into a space therethrough or a hole for releasing air therethrough is basically formed as a cavity covering the entire periphery of an area corresponding to the core. In the mold casting method, ceramic slurry 16 is injected into the metal mold 12 from the opening for injecting a material into the space of the metal mold 12 as indicated by an arrow 14. Specifically, a core 18 is manufactured by so-called injection molding while injecting the ceramic slurry 16 into the metal mold 12. In the mold manufacturing method, when the core 18 is manufactured inside the metal mold 12, the core 18 is separated from the metal mold 12 and the separated core 18 is baked while being disposed in a combustion furnace 20. Thus, the core 18 formed of ceramic is baked and hardened (Step S102).

In the mold manufacturing method, an outer metal mold is manufactured after the core 18 is manufactured (step S14). The outer metal mold is formed in a shape in which the inner peripheral surface corresponds to the outer peripheral surface of the cast metal. The metal mold may be formed of metal or ceramics. FIG. 6 is a schematic perspective view illustrating a part of the metal mold. In a metal mold 22 a illustrated in FIG. 6, a recess formed in the inner peripheral surface corresponds to the outer peripheral surface of the cast metal. Furthermore, only the metal mold 22 a is illustrated in FIG. 6, but another metal mold corresponding to the metal mold 22 a is also manufactured in a direction in which the recess formed on the inner peripheral surface is covered so as to correspond to the metal mold 22 a. In the mold manufacturing method, when two metal molds are combined with each other, a mold is obtained the inner peripheral surface of which corresponds to the outer peripheral surface of the cast metal.

In the mold manufacturing method, a wax pattern (a was mold) is manufactured after the outer metal mold is manufactured (step S16). Hereinafter, this process will be described with reference to FIG. 7. FIG. 7 is a schematic diagram illustrating a wax pattern manufacturing process. In the mold manufacturing method, the core 18 is disposed at a predetermined position of the metal mold 22 a (step S110). Subsequently, a metal mold 22 b corresponding to the metal mold 22 a covers the surface provided with the recess of the metal mold 22 a while surrounding the periphery of the core 18 by the metal molds 22 a and 22 b so that a space 24 is formed by the core 18 and the metal molds 22 a and 22 b. In the mold manufacturing method, wax 28 starts to be injected into the space 24 from a pipe connected to the space 24 as indicated by an arrow 26 (step S112). Here, the wax 28 is a material, for example, a wax, the melting point of which is comparatively low and which is heated and melted at a predetermined temperature or more. In the mold manufacturing method, the wax 28 is charged into the entire area of the space 24 (step S113). Subsequently, a wax pattern 30 in which the wax 28 surrounds the periphery of the core 18 is formed by hardening the wax 28. The wax pattern 30 is formed so that a portion basically formed of the wax 28 is formed in the same shape as the cast metal to be manufactured. Subsequently, in the cast metal manufacturing method, the wax pattern 30 is separated from the metal molds 22 a and 22 b and a sprue 32 is attached thereto (step S114). The sprue 32 is an opening into which molten metal as melted metal is input during casting. In the mold manufacturing method, the wax pattern 30 which has the core 18 therein and is formed of the wax 28 having the same shape as the cast metal is manufactured as described above.

In the mold manufacturing method, slurry applying (dipping) is performed after the wax pattern 30 is manufactured (step S18). FIG. 8 is a schematic diagram illustrating a configuration of applying slurry to a wax pattern. FIG. 9 is a schematic diagram illustrating an outer mold manufacturing process. In the mold manufacturing method, as illustrated in FIG. 8, the wax pattern 30 is immersed into a storage portion 41 filled with slurry 40 and is extracted so as to be dried (step S19). Thus, a prime layer 101A can be formed on the surface of the wax pattern 30.

Here, the slurry which is applied in step S18 is slurry directly applied to the wax pattern 30. As the slurry 40, slurry obtained by solely dispersing alumina ultrafine particles therein is used. In the slurry 40, it is desirable to use fireproof micro particles, for example, zirconia of about 350 mesh as flour. Further, it is desirable to use polycarboxylic acid as a dispersing agent. Further, it is desirable to add a small amount of a defoaming agent (a silicon material) or a wettability improving agent by, for example, 0.01% to the slurry 40. When the wettability improving agent is added to the slurry, it is possible to improve the adhesion property of the slurry 40 to the wax pattern 30.

In the mold manufacturing method, as illustrated in FIG. 8, slurry applying (dipping) is performed on the wax pattern having the prime layer (the first dry film) 101A by applying and drying the slurry 40 (step S20). Subsequently, as illustrated in FIG. 9, stuccoing is performed in which zircon stucco grains (having an average particle diameter of 0.8 mm) as a stucco material 54 are sprinkled to the surface of the wet slurry (step S21). Subsequently, a layer of the wet slurry in which the stucco material 54 attached to the surface thereof is dried so as to form a first back-up layer (a second dry film) 104-1 on the prime layer (the first dry film) 101A (step S22).

It is determined whether to repeat the process of forming the first back-up layer (the second dry film) 104-1 a plurality of times (for example, n: 6 to 10 times) (step S23). The n-th back-up layer 104-n is laminated a predetermined number of times (n) (step S23: Yes), so that a dried compact 106A is formed as an outer mold with the thickness of a multi-layered back-up layer 105A of, for example, 10 mm.

In the mold manufacturing method, when a dried compact 106A provided with a plurality of layers of the prime layer 101A and the multi-layered back-up layer 105A is obtained, a heat treatment is performed on the dried compact 106A (step S24). Specifically, the wax between the outer mold and the core is removed, and the outer mold and the core are sintered. Hereinafter, this will be described with reference to FIG. 10. FIG. 10 is a schematic diagram illustrating a part of the process of the mold manufacturing method. In the mold manufacturing method, as illustrated in step S130, the dried compact 106A as the outer mold provided with a plurality of layers of the prime layer 101A and the multi-layered back-up layer 105A is heated while being disposed inside an autoclave 60. The autoclave 60 heats the wax pattern 30 inside the dried compact 106A by charging pressurized steam thereinto. Thus, the wax forming the wax pattern 30 is melted so that melted wax 62 is discharged from a space 64 surrounded by the dried compact 106A.

In the mold manufacturing method, since the melted wax 62 is discharged from the space 64, a mold 72 is manufactured in which the space 64 is formed in an area filled with the wax between the core 18 and the dried compact 106A as the outer mold as illustrated in step S131. Subsequently, in the mold manufacturing method, the mold 72 provided with the space 64 between the core 18 and the dried compact 106A as the outer mold is heated by a combustion furnace 70 as illustrated in step S132. Thus, an unnecessary component or a moisture component included in the dried compact 106A as the outer mold is removed from the mold 72, and the mold is sintered and hardened so that the outer mold 61 is formed. In the cast metal manufacturing method, the mold 72 is manufactured as described above.

Referring to FIGS. 3 to 11, the casting method will be continuously described. FIG. 11 is a schematic diagram illustrating a part of a process of the casting method. In the casting method, the mold is preheated after the mold is manufactured in step S1 (step S2). For example, the mold is disposed in a furnace (a vacuum furnace and a combustion furnace) and is heated in the range of 800° C.-900° C. Due to the preheat treatment, it is possible to suppress the breakage of the mold when molten metal (melted metal) is injected into the mold to manufacture cast metal.

In the casting method, pouring is performed after the mold is preheated (step S3). That is, as illustrated in step S140 of FIG. 11, molten metal 80, that is, a raw material (for example, steel) of melted cast metal is injected from the opening of the mold 72 into a gap between the outer mold 61 and the core 18.

In the casting method, the outer mold 61 is removed after the molten metal 80 poured into the mold 72 is solidified (step S4). That is, when the cast metal 90 is obtained by solidifying the molten metal inside the mold 72 as illustrated step S141 of FIG. 11, the cuter mold 61 is milled so as to be separated as a fragment 61 a from the cast metal 90.

In the casting method, the core removal process is performed after the outer mold 61 is removed from the cast metal 90 (step S5). That is, as illustrated in step S142 of FIG. 11, a core removal process is performed by inserting the cast metal 90 into an autoclave 92. Then, the core 18 inside the cast metal 90 is melted, and a melted core 94 is discharged from the inside of the cast metal 90. Specifically, the cast metal 90 is dipped in an alkali solution inside the autoclave 92 and is pressurized and depressurized repeatedly so as to discharge the melted core 94 from the cast metal 90.

In the casting method, a finishing treatment is performed after the core removal process is performed (step S6). That is, a finishing treatment is performed on the surface or the inside of the cast metal 90. Further, in the casting method, the quality of the cast metal is checked along with the finishing treatment. Thus, a cast metal 100 can be manufactured as illustrated in step S143 of FIG. 11.

In the casting method of the embodiment, the cast metal is manufactured by manufacturing the mold by the use of the lost-wax casting method using wax as described above. Here, in the mold manufacturing method, the casting method, and the mold of the embodiment, the outer mold as the outer portion of the mold is formed as a multi-layer structure in which the prime layer (the first dry film as the prime layer) 101A is formed as the inner peripheral surface by using alumina ultrafine particles as slurry and the multi-layered back-up layer 105A is formed on the outside of the prime layer 101A.

As the precision-cast product according to the invention, for example, a gas turbine stater vane, a gas turbine combustor, a gas turbine split ring, or the like can be exemplified other than the gas turbine blade.

EXAMPLE 2

Next, a second precision-casting core will be described. FIG. 13 is a cross-sectional configuration diagram of a precision-casting core.

A precision-casting core according to the invention is obtained by forming a coating layer of two kinds of silica materials having different particle diameters on a surface of a sintered precision-casting core body (hereinafter, referred to as a “core body”) mainly including silica particles.

As illustrated in the upper stage of the cross-sectional view of the core body as the sintered body illustrated in FIG. 13, a plurality of holes 18 c is formed in a surface 18 b of a core body 18 a during sintering.

In the invention, as illustrated in the lower stage of FIG. 13, the holes 18 c formed in the surface 18 b are sealed by coating the holes 18 c by a coating layer 19 a.

In this example, the coating layer 19 a is formed on the surface 18 b of the core body 18 a as the sintered body by mixing and sintering silica particles obtained in Example 1 with silica fume (having a particle diameter of 0.15 μm).

As an example, two kinds of silica materials having different particle diameters are used in the coating layer 19 a.

Here, in two kinds of silica materials having different particle diameters, silica sol (SiO₂ of 30 wt %) is used as a first material and silica fume (having a particle diameter of 0.15 μm) is used as a second material.

In the invention, silica sol-silica fume slurry is prepared by adding and dispersing silica fume into silica sol.

Here, silica sol and silica fume are mixed at the weight ratio of 1:1 to 4:1. In the ratio of silica micro particles in silica sol in the silica sol-silica fume slurry kneaded at the weight ratio of 2:1, a sol solid content and silica fume were set as the ratio of 30:50.

The sintered core body 18 a is immersed into the obtained silica sol-silica fume slurry, and is pulled up later so as to form the coating layer 19 a formed of silica sol-silica fume on the surface of the core body 18 a. When the coating layer 19 a is formed, the slurry component also intrudes into the holes 18 c of the surface of the core, and hence silica sol-silica fume component is also precipitated in the holes of the core after the core body is dried.

Subsequently after the drying process, a heat treatment is performed at, for example, 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

In the obtained coating layer 19 a, since a gap of silica fume having a large particle diameter is filled by silica sol having a small particle diameter in the silica material as a constituent material, a compact layer is formed by the finely filling state.

Further, since silica fume has a spherical shape, silica sol having a small particle diameter easily intrudes into a gap of silica fume particles having a large particle diameter, and hence the silica material is further finely filled. Furthermore, since silica sol of micro particles improves the adhesion strength among the particles, they contributes to the improvement in strength.

In this way, according to the invention, the high-temperature strength of the precision-casting core is improved.

Further, a silica material and an alumina material are used as a second material forming the coating layer.

Here, the silica material is silica sol (SiO₂ of 30 wt %), and the alumina material is alumina sol (Al₂O₃).

Silica sol (SiO₂) and alumina sol (Al₂O₃) are mixed at the molar ratio of 2:3 so as to prepare mixed sol (silica-alumina sol).

A core sample is immersed into the prepared silica-alumina sol and is pulled up so as to form a layer of silica-alumina sol on the surface 18 b of the core body 18 a and to precipitate silica-alumina sol even in the hole 18 c of the core surface.

Subsequently after the drying process, for example, a heat treatment is performed at 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

In the heat treatment, silica-alumina sol changes to mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Thus, it is possible to obtain the core 18 in which the core body 18 a is covered by the mullite coating layer 19 a.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, the high casting temperature can be handled.

Further, an alkoxide material is used as a third material forming the coating layer.

Here, the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.

Silicon ethoxide or silicon butoxide is used as silicon alkoxide, and ethanol or butanol is used as solvent.

Further, when two kinds of alkoxide are mixed, a mixed alkoxide material obtained by mixing silicon alkoxide and aluminum alkoxide is used, and for example, alcohol solvent such as butanol is used as solvent.

When mixed alkoxide is prepared, mixed alkoxide of silicon ethoxide and aluminum isopropoxide is dissolved in a butanol solution.

Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) is mixed at the molar ratio of 2:3 so as to prepare organic mixed alkoxide.

The core sample is immersed into prepared solely alkoxide or mixed alkoxide and is pulled up so as to form a silicon layer or a silicon-aluminum alkoxide layer on the surface 18 b of the core body 18 a and to precipitate a silicon component or silicon-aluminum alkoxide component even in the hole 18 c of the core surface thereof.

Since solely alkoxide or mixed alkoxide is dissolved in an alcohol solution during immersing, solely alkoxide or mixed alkoxide easily penetrates into the core body, and hence a good coating layer is formed thereon.

Subsequently after the drying process, a heat treatment is performed at, for example, 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

In the heat treatment, in the case of mixed alkoxide, the silicon-aluminum alkoxide layer changes to inorganic mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Thus, it is possible to obtain the core 18 in which the core body 18 a is covered by the mullite coating layer 19 a.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, a high casting temperature can be handled.

Further, an alkoxide-silica fume material including an alkoxide material and silica fume are used as a fourth material forming the coating layer.

Here, the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.

In inorganic silica fume, for example, a spherical material having a particle diameter of 0.15 μm is used.

Here, it is desirable that silica fume have a particle diameter of 0.05 to 0.5 μm.

The dispersion ratio of silica fume is set to 5 to 40 wt % and appropriately about 20 wt %.

Silicon ethoxide or silicon butoxide is used as silicon alkoxide, and ethanol or butanol is used as solvent.

Further, when two kinds of alkoxide are mixed, a mixed alkoxide material obtained by mixing silicon alkoxide and aluminum alkoxide is used, and for example, alcohol solvent such as butanol is used as solvent.

When mixed alkoxide is prepared, mixed alkoxide of silicon ethoxide and aluminum isopropoxide is dissolved in a butanol solution.

Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) is mixed at the molar ratio of 2:3 so as to prepare organic mixed alkoxide.

The core sample is immersed into prepared solely alkoxide or mixed alkoxide having silica fume dispersed therein and is pulled up so as to form a silicon layer or a silicon-aluminum alkoxide layer including silica fume on the surface 18 b of the core body 18 a and to precipitate a silicon layer or a silicon-aluminum alkoxide component including silica fume even in the hole 18 c of the core surface.

Since solely alkoxide or mixed alkoxide is dissolved in an alcohol solution during immersing, solely alkoxide or mixed alkoxide easily penetrates into the core body, and hence a good coating layer is formed thereon.

Subsequently after the drying process, a heat treatment is performed at, for example, 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

After the drying process, alkoxide and silica fume components are precipitated even in the hole 18 c of the surface 18 b of the core body 18 a. At this time, a mixed layer is forced by a silica fume layer having a large particle size and a fine and compact alkoxide layer.

Then, since inorganic ceramic is generated in the alkoxide layer due to the heat treatment at 1,000° C., the gap of the silica fume layer having a large particle size is filled by a compact ceramic layer, and hence the adhesion strength among the particles is improved.

In the heat treatment, in the case of mixed alkoxide, the silicon-aluminum alkoxide layer including silica fume changes to inorganic mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Since the gap of the silica fume layer having a large particle size is filled by a compact mullite layer, it is possible to obtain the core 18 in which the core body 18 a is covered by the coating layer 19 a having the improved adhesion strength among particles.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, the high casting temperature can be handled.

In this way, according to the invention, since the plurality of holes formed in the surface is sealed, it is possible to prevent a problem in which the core is broken during casting from the holes as the start points in the related art. Accordingly, the high-temperature strength of the precision-casting core is improved.

Further, since silica fume has a large particle size, the heat shrinking is small even at the heat treatment of 1,000° C.

Further, as a fifth material forming the coating layer, a silica material, an alumina material, and silica fume are used.

Here, the silica material is silica sol (SiO₂ of 30 wt %) and the alumina material is alumina sol (Al₂O₃).

Here, the dispersion ratio of silica fume dispersed in the silica material and the alumina material is set to 5 to 40 wt % and appropriately about 20 wt %.

It is desirable that silica fume have a particle diameter of 0.05 to 0.5 μm.

Here, silica sol (SiO₂) and alumina sol (Al₂O₃) are mixed at the molar ratio of 2:3 so as to prepare mixed sol (silica-alumina sol) (in which the particle diameters of dispersed particles are from 1 to several hundreds of nanometers).

Silica fume is added and dispersed in the prepared silica-alumina sol so as to prepare silica-alumina sol-silica fume slurry.

The core sample is immersed into the prepared silica-alumina sol-silica fume slurry and is pulled up so as to form a layer of silica-alumina sol-silica fume on the surface 18 b of the core body 18 a and to precipitate a silica-alumina sol-silica fume component even in the hole 18 c of the core surface.

Subsequently after the drying process, a heat treatment is performed at, for example, 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

In the heat treatment, silica-alumina sol changes to mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Since the gap of the silica fume layer having a large particle size is filled by the compact mullite layer, it is possible to obtain the core 18 in which the core body 18 a is covered by the coating layer 19 a having the improved adhesion strength among particles.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, the high casting temperature can be handled.

In this way, according to the invention, since the plurality of holes formed in the surface is sealed, it is possible to prevent a problem in which the core is broken during casting from the holes as the start points in the related art. Accordingly, the high-temperature strength of the precision-casting core is improved.

Further, since silica fume has a large particle size, the heat shrinking is small even at the heat treatment of 1,000° C.

TEST EXAMPLE 4

Hereinafter, a test example for verifying the effect of the invention will be described.

In the test example, a compound was obtained by adding wax to a mixture in which silica fume of 20 wt % was added to silica sand (220 mesh) and heating and kneading the mixture.

A compact was obtained by injection molding the obtained compound.

As a test sample, a sample having a width of 30 mm, a length of 200 mm, and a thickness of 5 mm was obtained.

Next, a sample for a core body was obtained by performing a decreasing treatment to 600° C. and a sintering treatment at 1,200° C.

Next, silica sol-silica fume slurry was prepared (silica sol and silica fume were kneaded at the weight ratio of 2:1) by adding and dispersing silica fume (having a particle diameter of 0.15 μm and a spherical shape) into silica sol (SiO₂ of 30 wt %). At this time, in the ratio of silica micro particles in silica sol, a sol solid content and silica fume were set as the ratio of 30:50.

The sample for the core body was immersed into the obtained silica sol-silica fume slurry and was pulled up so as to form the coating layer 19 a of silica sol-silica fume on the surface. Subsequently after the drying process, a heat treatment was performed at 1,000° C. so that the coating layer 19 a formed of silica sol-silica fume was formed on the surface 18 b of the core body 18 a.

As a comparative example, a core body without a coating layer was prepared as a comparative sample.

The strength of each of the test samples was measured.

Here, the strength test was performed based on “Bending Strength of Ceramics (1981)” of JIS R 1601.

The strength of the comparative sample without the coating layer according to the conventional method was 23 MPa, but, to the contrary, the strength of the sample for the core body according to the method of the invention was 29 MPa. As a result, in the sample for the core body of the invention, it was acknowledged that the strength was improved by 25%.

Hereinafter, a casting method using a mold having the precision-casting core of the invention disposed therein will be described.

Furthermore, the same process as that of the casting method of Example 1 will be omitted, and only the “core manufacturing process” will be described with reference to FIG. 12.

FIG. 12 is a schematic diagram illustrating a core manufacturing process according to Example 2. In the mold, manufacturing method, as illustrated in FIG. 12, the metal mold 12 is prepared (step S101). The metal mold 12 is formed so that an area corresponding to the core is formed as a cavity. A portion formed as the cavity of the core corresponds to the convex portion 12 a. Furthermore, although it is illustrated in the cross-section of the metal mold 12 in FIG. 12, the metal mold 12 is formed so that an area other than an opening for injecting a material into a space therethrough or a hole for releasing air therethrough is basically formed as a cavity covering the entire periphery of an area corresponding to the core. In the mold casting method, ceramic slurry 16 is injected into the metal mold 12 from the opening for injecting a material into the space of the metal mold 12 as indicated by an arrow 14. Specifically, a core 18 is manufactured by so-called injection molding while injecting the ceramic slurry 16 into the metal mold 12. In the mold manufacturing method, when the core 18 is manufactured inside the metal mold 12, the core 18 is separated from the metal mold 12 and the separated core 18 is baked while being disposed in a combustion furnace 20. Thus, the core 18 formed of ceramic is baked and hardened (step S102).

The processes described so far are the same as those of Example 1.

Subsequently, as illustrated in FIG. 12, in order to form a coating layer on the surface of the core 18, the sintered core 18 is immersed into a storage portion 17 filled with the slurry 19 and is extracted so as to be dried (step S103). Subsequently, the immersed core 18 is extracted and is baked while being disposed in the combustion furnace 20. Thus, the coating layer 19 a is formed on the surface of the core 18 formed of ceramic (step S104).

In the mold casting method, the core 18 provided with the coating layer 19 a is manufactured as described above. Furthermore, the core 18 is formed of a material which can be removed after the cast metal is hardened by a core removal process such as a chemical treatment.

Since the coating layer is formed on the surface of the core in the casting method of the embodiment, the dimensional precision is improved, and hence the durability is improved even at the high casting temperature.

Further, since the high-strength core is provided, the degree of freedom in design (for example, a slow pulling-down speed or the like) of the casting is improved even at the long casting process time.

Furthermore, it is possible to provide a precision-cast product such as a turbine blade which is thin and has good thermal efficiency.

REFERENCE SIGNS LIST

12, 22 a, 22 b METAL MOLD

12 a CONVEX PORTION

14, 26 ARROW

16 CERAMIC SLURRY

18 CORE

18 a CORE BODY

18 b SURFACE

18 c HOLE

19 SLURRY

19 a COATING LAYER

20, 70 COMBUSTION FURNACE

24, 64 SPACE

28 WAX

30 WAX PATTERN

32 SPRUE

40 SLURRY

60, 92 AUTOCLAVE

61 OUTER MOLD

61 a FRAGMENT

62 MELTED WAX

72 MOLD

80 MOLTEN METAL

90, 100 CAST METAL

94 MELTED CORE

101A PRIME LAYER 

1. A precision-casting core in which a precision-casting core body is formed by mixing and sintering silica particles of silica sand of 20 to 70 μm and silica fume of 5 wt % to 30 wt % as additive amount.
 2. The precision-casting core according to claim 1, wherein a coating layer is formed on the surface of the sintered precision-casting core body.
 3. A precision-casting mold used to manufacture a cast metal, comprising: a precision-casting core in which a precision-casting core body is formed by mixing and sintering silica particles of silica sand of 20 to 70 μm and silica fume of 5 wt % to 30 wt % as additive amount having a shape corresponding to a cavity inside the cast metal; and an outer mold corresponding to the shape of the outer peripheral surface of the cast metal.
 4. A precision-casting core manufacturing method comprising: immersing a sintered body of a precision-casting core body mainly including silica particles of silica sand of 20 to 70 μm into a coating material including a silica material and an alumina material; drying the sintered body; and heating the sintered body so as to form a coating layer on the surface of the precision-casting core body.
 5. The precision-casting core manufacturing method according to claim 4, wherein the silica material is silica sol and the alumina material is alumina sol.
 6. The precision-casting mold according to claim 3, wherein a coating layer is formed on the surface of the sintered precision-casting core body.
 7. The precision-casting core according to claim 2, wherein silica sol and silica fume are used as a material forming the coating layer.
 8. The precision-casting core according to claim 2, wherein an alkoxide material is used as a material forming the coating layer.
 9. The precision-casting core according to claim 2, wherein an alkoxide-silica fume material are used as a material forming the coating layer.
 10. The precision-casting mold according to claim 6, wherein silica sol and silica fume are used as a material forming the coating layer.
 11. The precision-casting mold according to claim 6, wherein an alkoxide material is used as a material forming the coating layer.
 12. The precision-casting mold according to claim 6, wherein an alkoxide-silica fume material are used as a material forming the coating layer. 